Abstract
The adsorption characteristics of protoxin and toxin from Bacillus thuringiensis by sepiolite were studied. The kinetic results showed that the protoxin and toxin could be adsorbed by sepiolite rapidly, and the equilibrium was reached within 1 h. The adsorption isotherms of both proteins followed Langmuir equation (R2>0.98) and the curves belonged to L type. The adsorption capacity, adsorption rate and the insecticidal activity of toxin were higher than those of protoxin. The results of ultraviolet irradiation showed that sepiolite could protect the toxin from ultraviolet irradiation damage, but it accelerated the damage of protoxin. Therefore, the toxin is suitable for preparing long-term formulations of B. thuringiensis using the carrier of sepiolite. The results of Fourier transform infrared, X-ray diffraction, and scanning electron microscopy indicated that toxin did not influence the structure of sepiolite and the adsorption of toxin only on the surface of sepiolite. The adsorption of toxin by sepiolite was enhanced by metal ions in the range of concentration from 0 to 4.0 mmol L−1, and the enhancement extent of metal ions on the adsorption of toxin was in the order: Ca2+>Mg2+>K+>Na+. The higher the loading of toxin per mass sepiolite, the lower the 50% of the larvae of sepiolite–toxin complex. The zeta potential of sepiolite suspension increased with the addition of metal ions, indicating that the driving forces of the adsorption of toxin to negatively charged sepiolite was electrostatic interaction.
Highlights
Bacillus thuringiensis (Bt) has been extensively used for five decades in biopesticidal formulations due to its safe environmental and human health records
The present study indicated that both adsorption amount and insecticidal activity of toxin were higher than those of protoxin, the toxin is suitable for preparing long-term formulations of B. thuringiensis using the carrier of sepiolite
The protoxin and toxin from B. thuringiensis could be adsorbed by sepiolite rapidly, and the equilibrium was reached within 1 h
Summary
Bacillus thuringiensis (Bt) has been extensively used for five decades in biopesticidal formulations due to its safe environmental and human health records. It is cultured in commercially available liquid media and spray-dried to produce high concentrations of raw materials (technical powder) including mainly crystal proteins and endospores for formulation (Brar et al, 2006; Zhou et al, 2004). The adsorption or immobilization of insecticidal proteins on clay minerals may provide a new approach for the preparation of long-term formulations of B. thuringiensis, available data showed that the loading capacity of toxin on minerals was low. Some new measurements for improving the adsorption efficacy of Bt toxin onto clay minerals deserves further studies
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